I don't know about guidelines - maybe someone else does, but I agree that it seems rather absurd to go ahead with a medical without an ESA50. On the other hand, given that the medical seems to consist of standard questions leading to tick-box answers on a computer screen, requiring no relevant clinical knowledge, I suppose that it can be argued that they don't need to see the ESA50 either because it is a standardised process.
If you think that you fall into one of the categories that shouldn't need a medical, have a medical and feel that there are circumstances that haven't been taken into account in the medical, all you can really do is to make sure the Decision Maker has as much evidence as possible on which to make their decision. It's not ideal, though.